Reading recorded information from an optical information recording medium (optical disc) such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) is conducted by converging a light beam emitted from a light source such as a semiconductor laser device on a recording track of the optical disc by using an objective lens and converting reflected light from the optical disc to an electric signal by a photodetector. In order to accurately converge a light beam on a desired recording track of a rapidly spinning optical disc, a focus error signal and a tracking error signal are detected and the position of the objective lens is controlled according to surface displacement, eccentricity, and the like of the optical disc.
A differential push-pull (DPP) method is known as a typical method for detecting a tracking error signal. In the DPP method, a light beam is separated into three beams: a main beam; a +1st order diffracted beam; and a −1st order diffracted beam. These three beams are respectively converged on three adjacent guide grooves formed at a prescribed pitch on the optical disc. Push-pull signals respectively obtained by detecting reflected light of the three beams and performing an arithmetic operation have a phase difference of 180 degrees between the main beam and the +1st and −1st order diffracted beams. Therefore, by performing arithmetic processing of each push-pull signal, only offset components included in the push-pull signals are selectively cancelled each other, whereby an excellent tracking error signal can be detected. Accordingly, the DDP method has been widely used especially in a DVD recording optical pickup (e.g., see Patent document 1).
There are various standards for currently used optical discs, and a guide groove pitch varies depending on the standards of the optical discs. For example, optical discs such as a write once type DVD-R (Recordable) and an erasable type DVD-RW (Disk ReWritable) have a guide groove pitch of 0.74 μm, and optical discs such as an erasable type DVD-RAM (Random Access Memory) has a guide groove pitch of 1.23 μm. An optical pickup device that enables recording and playback of two or more types of optical discs of different standards has been demanded. The following optical pickup device is proposed in view of this demand (e.g., see Patent document 2).
In the optical pickup device disclosed in Patent document 2, a special diffraction grating for separating a light beam is divided into three regions, and the phase of grating grooves periodically provided in each region is sequentially shifted by 90 degrees. A tracking error detection method using such a special diffraction grating is called an in-line DPP method, and the in-line DPP method enables stable tracking error detection on a plurality of optical information recording media having different guide groove pitches.
Patent document 1: Japanese Patent Publication for Opposition No. 4-34212
Patent document 2: Japanese Laid-Open Patent Publication No. 2004-145915